Recently, particularly in mobile phones among portable information processing apparatuses, sets are being downsized and thinned with multi-functionalization such as installation of cameras, stereo reproduction of ringtone melodies, and the like, and with size increase of liquid crystal screens. This makes it difficult to obtain a space for placing a loudspeaker within a mobile phone set, and there is a demand for downsizing a receiver which is a loudspeaker for reproducing a reception sound of a mobile phone and downsizing a micro loudspeaker for reproducing a ringtone and/or a music signal.
In addition, not only mobile phones but also video devices, such as flat-screen televisions, including PDPs (Plasma Display Panels), liquid crystal panels, or the like are desired to be further thinned and frame-narrowed by narrowing outer frames enclosing the PDPs or the liquid crystal panels to a maximum extent in order to make screens appear larger according to a design demand. Therefore, loudspeakers installed in the flat-screen televisions are also desired to have small and slim shapes with narrow widths.
Further, in a receiver of a mobile phone, for example, as a mobile phone system advances from the third generation to the fourth generation, expansion of a low frequency range for a wider reproduction range is also desired.
As described above, downsizing and widening of a reproduction range are demanded in loudspeakers of mobile phones and flat-screen televisions.
In a conventional loudspeaker structure, however, downsizing causes a support (an edge, a damper, and the like) which vibratably supports a diaphragm to be reduced in width and accordingly to be increased in stiffness. As a result, the lowest resonance frequency of the loudspeaker becomes higher to fail to expand a low frequency range, which makes it difficult to widen a reproduction range. Thus, in the conventional loudspeaker structure, it is difficult to realize both downsizing and widening of a reproduction range.
As one of prior arts for solving this problem, a loudspeaker disclosed in Patent Document 1 has been proposed. FIG. 15 is a structure section of a conventional loudspeaker disclosed in Patent Document 1. In FIG. 15, a loudspeaker includes a yoke 1, a magnet 2, a plate 3, a voice coil 4, a diaphragm 5, a butterfly damper 6, a magnet 7, and a magnetic fluid 8.
The magnet 2 is fixed to an inner bottom surface of the yoke 1 which has a box-like shape with an upper surface thereof opened. The plate 3 is fixed to an upper surface of the magnet 2. A magnetic gap G1 is formed between the yoke 1 and the plate 3. Thus, the yoke 1, the magnet 2, and the plate 3 constitute a magnetic circuit having the magnetic gap G1. The voice coil 4 is provided on the outer circumference of the diaphragm 5 and positioned within the magnetic gap G1. The butterfly damper 6 is a support that vibratably supports the diaphragm 5, and provided on the outer circumference of the diaphragm 5. The magnet 7 is provided on the outer circumference of the diaphragm 5. The magnetic fluid 8, which is held by magnetic force of the magnet 7, vibrates the diaphragm 5 in a stable manner because of its viscosity.
Operations and effects of the loudspeaker shown in FIG. 15 and configured as above will be described. When a music signal is applied to the voice coil 4, the diaphragm 5 vibrates and a sound is emitted from the diaphragm 5. Here, the stiffness of the butterfly damper 6 is, because of a structure thereof, smaller than the stiffness of an ordinary support (an edge, a damper, and the like). Therefore, in the loudspeaker shown in FIG. 15, even when the loudspeaker is downsized, the lowest resonance frequency of the loudspeaker can be lowered to enable a reproduction range to be widened.
Further, even when the loudspeaker is downsized so that the width of the butterfly damper 6 is reduced, the butterfly damper 6 can show high stroke performance by increasing a length of a joint portion of the butterfly damper 6. Therefore, in the loudspeaker shown in FIG. 15, non-linear distortion by the support, which is caused in the case of large amplitude of the diaphragm 5, can be reduced.
As described above, the loudspeaker shown in FIG. 15 realizes both of downsizing of the loudspeaker and widening of a reproduction range, and moreover reduces non-linear distortion by the support.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-274206